Erbium doped fiber amplifiers (EDFAs) are used extensively alone or in subsystems to amplify fiber optic signals in single channel and dense wavelength division multiplexing (DWDM) optical networks. The EDFA has the capability of passing energy from a “pump” laser to the optical signal to be amplified. The gain of the EDFA is a function of the input, the pump power and their corresponding history (e.g., over the previous milliseconds).
Subsystems and module products available in recent years contain two or more EDFAs cascaded together and separated by a dispersion compensation module (DCM) which can by the nature of its design also introduce a delay. For example, FIG. 1 shows a conventional two-stage cascaded optical amplifier 14. An add/drop type optical signal is input to a first EDFAa and its amplified output is input to a second EDFAb. The output of EDFAa is coupled to the input of EDFAb via a DCM with its corresponding delay. The output of EDFAb represents the output of the cascaded amplifier.
A first control algorithm 20 provides gain control of EDFAa. A power coupler or tap 22 senses the power of the optical signal input to EDFAa and provides a control input PINa1 to the control algorithm 20. Similarly, a tap 24 senses the power of the amplified optical signal output by EDFAa and provides a control input PINa2 to the control algorithm. The control algorithm 20 compares the output power of EDFAa to the input power of EDFAa. Based on the desired gain of EDFAa, the control algorithm 20 provides a gain control signal to EDFAa in the form of a pump control signal to Pump a. By controlling the laser pump energy delivered by Pump a, the control algorithm 20 controls the gain provided by EDFAa.
The amplified optical signal output from EDFAa is coupled to the input of EDFAb via a DCM 26. EDFAb in turn further amplifies the optical signal output from EDFAa. A second control algorithm 30 serves to provide gain control of EDFAb. Specifically, a tap 32 outputs a control input PINb1 indicative of the power of the input signal to EDFAb, and a tap 34 provides a control input PINb2 indicative of the power of the optical signal output by EDFAb. The control algorithm 30 receives the control inputs PINb1 and PINb2 and based thereon compares the input and output signal power of EDFAb with the desired gain. Based on such comparison, the control algorithm 30 controls the laser pump energy delivered by Pump b, which in turn controls the gain of EDFAb.
Cascaded optical amplifiers such as that shown in FIG. 1 have generally provided satisfactory results. However, there have been certain drawbacks or disadvantages that have led to less than optimum performance. For example, errors or noise introduced by amplifiers upstream tend to accumulate and are exaggerated by amplifiers downstream in the cascade.
In view of the aforementioned shortcomings associated with existing cascaded optical amplifiers, there is a strong need in the art for a cascaded amplifier that is less prone to the accumulation of errors and/or noise. Moreover, there is a strong need in the art for a cascaded amplifier in which downstream amplifiers exhibit an improved dynamic response.